The equivalence principle is a basic premise that is essential to every experimentally verified physical theory, including General Relativity and the Standard Model. It states that an object's inertial mass is equivalent to its gravitational mass. The inertial mass of an object appears in Newton's second law: the force applied to the object is equal to its mass times its acceleration. The gravitational mass of an object is the gravitational equivalent of electric charge: the physical property of an object that causes it to interact with other objects through the gravitational force. There is no a priori reason to assume that these two types of "mass" are the same, but experiments have verified that the equivalence principle holds to a part in 1013.